1. Field of the Invention
The present invention relates to a method of halftoning an image, and, more particularly, to halftoning using multiple weight error diffusion.
2. Description of the Related Art
In recent years many printers have been developed for home and office use. These printers are often used to reproduce continuous tone images displayed on other devices, such as computer monitors. Such continuous tone images may include scanned images, photos downloaded from the internet or from a digital camera, as well as images created or modified by a user with various application software products that are available to businesses and consumers alike. In order to reproduce continuous tone images on a printer, a process of halftoning must be performed on the image data before the image may be printed. Various halftoning methods are available, including dithering, blue noise masking, and error diffusion. These methods may be used individually or in combination, and are used in digital printing to convert multi-level i.e., gray level, input values to bi-level output values to be printed.
With the error diffusion method of halftoning, each input value associated with a pixel location in an image is compared with a threshold. If the input value along with any accumulated error is greater than the threshold, the pixel is turned on, i.e., printed, and an error, which is the difference between the input value and the maximum-input value of 255, is spread over to certain neighbors of the pixel, contributing to the accumulated error of those pixel neighbors. If input is smaller than the threshold, the pixel is not turned on, and the input value plus any accumulated error is spread over to certain neighbors of the pixel as error, contributing to the accumulated error of those pixel neighbors.
Prior are methods have not adequately addressed problems associated with a pixel grid that is not symmetric. For example, hardware limitations or efficiency considerations may require that pixels be placed in groups inside a grid. The grouping may sometimes break the symmetry of the grid, for example, where the pixels are placed in groups of two in a hexagonal grid. Under such circumstances, the neighboring pixels for each pixel in the group of two are different. In these cases, having the same threshold and spread ratio for all elements in the group of pixels may result in an output image that appears grainy.
What is needed in the art is a method of error diffusion that accommodates a pixel grid that is not symmetric.